A passive solar house that is losing heat to the outdoors at an average rate of 50,000

Question:

A passive solar house that is losing heat to the outdoors at an average rate of 50,000 kJ/h is maintained at 22oC at all times during a winter night for 10 h. The house is to be heated by 50 glass containers each containing 20 L of water that is heated to 80oC during the day by absorbing solar energy. A thermostat-controlled 15-kW back-up electric resistance heater turns on whenever necessary to keep the house at 22oC.
(a) How long did the electric heating system run that night?
(b) How long would the electric heater run that night if the house incorporated no solar heating?